linux-kernel - RE: [RFC PATCH v2 2/6] PM / Runtime: introduce pm_runtime_set_memalloc

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B7060@saturn3.aculab.com>
Date:	Wed, 24 Oct 2012 10:06:36 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Ming Lei" <ming.lei@...onical.com>,
	"Alan Stern" <stern@...land.harvard.edu>
Cc:	<linux-kernel@...r.kernel.org>, "Oliver Neukum" <oneukum@...e.de>,
	"Minchan Kim" <minchan@...nel.org>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, "Jens Axboe" <axboe@...nel.dk>,
	"David S. Miller" <davem@...emloft.net>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<netdev@...r.kernel.org>, <linux-usb@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, <linux-mm@...ck.org>
Subject: RE: [RFC PATCH v2 2/6] PM / Runtime: introduce pm_runtime_set_memalloc_noio()

> Looks the problem is worse than above, not only bitfields are affected, the
> adjacent fields might be involved too, see:
> 
>            http://lwn.net/Articles/478657/

Not mentioned in there is that even with x86/amd64 given
a struct with the following adjacent fields:
	char a;
	char b;
	char c;
then foo->b |= 0x80; might do a 32bit RMW cycle.
This will (well might - but probably does) happen
if compiled to a 'BTS' instruction.
The x86 instruction set docs are actually unclear
as to whether the 32bit cycle might even be misaligned!
amd64 might do a 64bit cycle (not checked the docs).

	David



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